European patent application No. 0 052 442 discloses heterocyclic compounds of the formula: ##STR2## Wherein either X is --CR.sup.1 R.sup.2 -- and Y is oxygen, sulphur or --NR.sup.3 --, wherein R.sup.1, R.sup.2 and R.sup.3, which may be the same or different, each is hydrogen or alkyl of up to 4 carbon atoms; or X is oxygen, sulphur or --NH-- and Y is --CH.sub.2 --; wherein R.sup.4 and R.sup.5, which may be the same or different, each is hydrogen, cyano, nitro, amino or hydroxy, or alkylthio of up to 4 carbon atoms, or has various other meanings defined in claim 1, provided that R.sup.4 and R.sup.5 are not both hydrogen; or wherein R.sup.4 and R.sup.5 are joined together such that with the benzene ring A they form a benzheterocyclic ring as defined in claim 1; and therein the benzene ring A may optionally bear one or more further substituents; or a salt thereof where appropriate.
These compounds possess cardiotonic properties, and some of them possess peripheral vasodilator properties.
On the other hand a compound of most interest under clinical investigation at present as a cardiotonic agent is 1,6-dihydro-2-methyl-6-oxo-[3,4'-bipyridine]-5-carbonitrile known by the name Milrinone, which has the structure: ##STR3##